The present invention relates to the testing of data-compressed video systems, and more particularly to spatial bandwidth testing for digital data-compressed video systems for analyzing distortion of transmitted video images resulting from the data compression.
Data-compressed video systems, such as video coders/decoders (codecs), use data compression techniques in the transmission of video images to meet the bandwidth limitations of the transmission medium. Analog systems suffer from well understood phenomena, such as gain error, high frequency roll-off, group delay error and non-linear effects. These distortions are a function neither of the signal being transmitted nor of the signals previously transmitted, and they are amenable to analysis with traditional test equipment. Digital data-compressed video systems, however, produce distortions that are a function both of the signal being transmitted and of signals previously transmitted. The distortions are a function of the spatial and temporal complexity of the transmitted image. Images with higher entropy, i.e., randomness or lack of predictability, generally suffer more distortion. Traditional test signals suffer little distortion since they have low entropy--such signals are highly correlated along at least one axis.
Test methods are needed that use high entropy signals. Unfortunately the very complexity of such signals makes them difficult to analyze numerically. Therefore most testing of digital data-compressed video systems has involved subjective testing of complex picture material by a group of trained observers. This testing is expensive, complex to set up and, as it requires a highly controlled environment and the presence of the trained team, is not easily reproduced at any arbitrary location.
Another recent attempt for testing digital data-compressed video systems is disclosed in U.S. patent application Ser. No. 07/663,220 filed Mar. 1, 1991 by Michael S. Overton et al entitled "Three-Dimensional Testing of Video Codecs." The test signal used is a conventional test signal over a complex background signal, such as a pseudo-random noise signal or a zoneplate signal. A conventional measurement system, such as a waveform monitor or oscilloscope, displays the conventional test signal, with distortions caused by the complex background signal appearing in the display. The measurement instrument may be a spectrum analyzer where the background signal is a zoneplate signal.
What is desired is spatial bandwidth testing for digital data-compressed video systems that uses a high entropy test signal to provide an objective characterization of system distortions.